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The false alarm rate can be assessed by calculating the percentage of subgroups that are deemed out-of-control when the process is in control.Ĥ To help evaluate how well the control charts are performing, the ASSISTANT Report Card automatically performs the following data checks: Stability Number of subgroups Subgroup size Expected Variation ATTRIBUTE CONTROL CHARTS 2 In this PAPER, we investigate how an attribute control chart behaves when these conditions vary and we describe how we established a set of guidelines to evaluate requirements for these conditions. A good control chart should also rarely signal a false alarm when the process is in control. This sensitivity can be assessed by calculating the average number of subgroups needed to signal a special cause. A good control chart should be sensitive enough to quickly signal when a special cause exists. The data collected are the total number of defects in each subgroup, which is assumed to follow a Poisson distribution with an unknown mean number of defects per subgroup.ģ The control limits for a control chart are typically set in the control phase of a Six Sigma project. The U chart plots the number of defects per unit. U chart: This chart is used when a product or service can have multiple defects and the number of defects is counted. The data collected are the number of defective items in each subgroup, which is assumed to follow a binomial distribution with an unknown proportion parameter (p). The MINITAB ASSISTANT includes two widely used control charts to monitor a process with attribute data: P chart: This chart is used when a product or service is characterized as defective or not defective.Ģ The P chart plots the proportion of defective items per subgroup. When it is not possible to measure the quality of a product or service with continuous data, attribute data is often collected to assess its quality. Attribute Control Charts Overview Control charts are used to regularly monitor a process to determine whether it is in control. 1 MINITAB ASSISTANT WHITE PAPER This PAPER explains the research conducted by MINITAB statisticians to develop the methods and data checks used in the ASSISTANT in MINITAB statistical software.